Mealybugs are from the Pseudoccidae family of insects and can be one of the most difficult pests to control in plant collections. Several different species are now present in the United Kingdom. Some of these species can be quite difficult to identify. They are called Mealybugs because the females are covered in a white wax like material. The optimum temperature for Mealybugs development is from 18-24.c. Mealybugs are mobile, and their crawlers will search for places to feed and establish. High nitrogen content of plants will speed up their reproduction, some species mate and lay eggs and others reproduce asexually.
The three main species found in UK glasshouses are;
- Citrus Mealybug [ Planococcus citri ] – the filaments on the body are very short, yellow eggs attached to the abdomen with a dorsal stripe
- Long tailed Mealybug [ Pseudoccus longispnius ] – filaments longer than body. No eggs as viviparous, dorsal stripe not always visible.
- Obscure Mealybug [ Pseudoccus viburni / affinis ] – two filaments of medium length, no dorsal stripe and slightly darker than the Citrus Mealybug.
The nymphs and female adults feed on plant sap. Male Mealybugs do not feed, so do not cause plant damage. Once Mealybugs start feeding they excrete honeydew onto plants, this is messy and often leads to black moulds growing on it. The feeding can cause leaves to go yellow and fall off. Growth is reduced, and photosynthesis of the leaves is reduced. Plants will sometimes also drop their flowers. Some species can also transmit plant viruses. Mealybugs are mobile, and their crawlers will search for places to feed and establish on plants. High nitrogen content of plants will speed up their reproduction, some species mate and lay eggs and others reproduce asexually.
Species of Mealybug and how to control
This is the most difficult of the more common Mealybug species to control because it does not produce eggs. Cryptolaemus will feed on this species but are not as effective as they are against other species. This should be taken into consideration when deciding on introduction rates with this species possibly requiring higher inputs of Cryptolaemus. There is a parasitic wasp called Anagyrus fusciventris, which can be also introduced against Longtailed Mealybugs but this now requires a release license, as it is not a native parasite to the United Kingdom. Dragonfli and other biological control companies are applying for this.
A combination of Cryptolaemus and the parasitic wasp; Leptomastix epona can be used to control this Mealybug. Introduce the Leptomastix epona when temperatures are above 15.c, this wasp does prefer good light conditions and Cryptolaemus larvae in the form of CRYPTOBUG L when the environmental conditions are right for the Ladybird in the greenhouses. Leptomastix epona is 2-3mm long, with long black antennae, its wings have black stripes. Mummified Mealybugs turn yellow and the wasp emerges from a circular hole in the end.
Mealybugs are good at finding places to hide, especially in the winter months when they are less active. They will crawl into small spaces on plants or in greenhouse structures, so taking steps to reduce or eliminate Mealybug populations before the winter is good practice. This can be done by increasing biological control inputs in the summer when they are at their most efficient and by cleaning plants of the Mealybugs in the early autumn before they take refuge in hiding places. A good product for treating Mealybugs when the conditions are not suitable for biological control is SB Plant Invigorator.
The most capable biological control of Mealybugs is with the use of the predatory Ladybird; Cryptolaemus montrouzieri. This is a black Ladybird with an orange head, which especially likes feeding on Mealybug eggs. The larvae of this Ladybird will also feed on Mealybugs and even looks like a large Mealybug. Once Mealybugs are not present, the Cryptolaemus will often also feed on scale insects.
The optimal conditions for Cryptolaemus development are from temperatures between 22-25.c and in sunny conditions. Mature larvae can consume up to 30 Mealybugs a day. Introducing Cryptolaemus in high numbers once Mealybug is present is proven to be a better approach than drip feeding low amounts on a regular basis. The product CRYPTOBUG L, which contains up to 1000 larvae, enables large numbers to be put onto plants quickly. This can be distributed to large plants or in accessible plants with small distribution boxes, which the CRYPTOBUG L contents are poured into and then hung on the branches or stems of plants. The larvae then crawl out of the D-boxes. By introducing larvae, this ensures the biological control can be directed straight onto the Mealybug mass and is unable to fly off unlike adult Ladybirds. About eight D-boxes are needed to distribute one bottle of CRYPTOBUG L. Depending on the severity of the Mealybug infestation, introductions should be made 2-3 x in the spring and summer. The Cryptolaemus larvae will then develop into adults, which in turn also feed on Mealybugs.
Citrus Mealybug also has a parasitic wasp that can be introduced to control older Mealybugs. This is called; Leptomastix dactylopi and is a small yellow parasitic wasp with long bent antennae. It is tolerant of lower temperatures and humidity. It can be introduced from March-September, if temperatures are above 17.c. The mummified Mealybugs turn yellow and become hard, the wasp emerges from a circular hole in the end of the mummy. It can be good practice to introduce these wasps in the spring before the conditions are suitable for Cryptolaemus.
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